High Aluminum Ordering in SSZ-59: Residual 1H-27Al Dipolar Coupling Effects in 1H MAS NMR Spectra of Bronsted Acid Sites in Zeolites

The extra-large pore zeolite SSZ-59 was prepared with highly ordered Al atoms in T6 positions within the double-zigzag chains (dzc) running along the 14 MR channels. Narrow MAS NMR signals of bridging OH groups, which are responsible for Bronsted acidity, emerge from this Al site ordering. The theoretically predicted residual H-1-Al-27 dipolar effect that is caused by the large Al-27 quadrupolar coupling associated with the trigonally distorted Al coordination near this OH group was observed here with an unprecedented spectral resolution. Three H-1 NMR signals are separately observed at B-0 = 7.0 T for proximal Al-27 spins with magnetic quantum numbers of m = +/- 1/2, m = +/- 3/2, or m = +/- 5/2. A short H-1-Al-27 distance of 2.52 +/- 0.02 A was measured by means of H-1{Al-27} rotational echo adiabatic passage double-resonance (REAPDOR) characteristic for a Bronsted acid site. This distance predominates the initial slope of the H-1{Al-27} REAPDOR evolution curve, whereas additional much weaker H-1-Al-27 dipolar interactions were found by a second, less steep slope at longer evolution times. The second slope can be modeled either by a second Al atom within the dzc chain located at a distance of 6.7 angstrom to the bridging OH group or, alternatively, by two Al atoms in the dzc, with H-Al distances of 8.09 and 8.72 angstrom. The latter model with two Al is in excellent agreement with the packing of the organic structure directing agent in the 14 MR pores of the as-made material, which places the heteroatoms-in this case boron atoms that are postsynthetically replaced by aluminum after calcination/removal of the organics-into a highly ordered pattern in the dzc. The data indicate that the packing of the organics in adjacent 14 MR channels is not independent and shifted by 1.5 unit cells along the a-direction.

Automated summary

The researchers successfully prepared the highly ordered extra-large pore zeolite SSZ-59 with Al atoms in T6 positions forming double-zigzag chains along the 14 MR channels. Analysis of NMR signals revealed the impact of Al site ordering on Bronsted acidity, marking the first observation of the residual H-1-Al-27 dipolar effect.